This application is a national stage of PCT/EP981/04408 filed Jul. 15, 1998 and based upon 97112724.6 filed Jul. 24, 1997 under the International Convention.
The invention relates to a flue gas pipe with a reaction zone and a heat exchanger arranged downstream thereof, as well as to a method of treating flue gas.
A flue gas pipe for the transport and treatment of flue or exhaust gases is used in a combustion installation, such as a power plant running on fossil fuel or a garbage incinerator.
In the reaction zone the flue gas is thermally or catalytically treated, in order to break down possible harmful substances. Also in a reaction zone of this type so-called adsorbents are used for the capture and storage of harmful substances. Such a reaction zone is for instance a reactor traversed by the flue gas with a specifically acting catalyst arranged therein.
In the downstream heat exchanger the flue gas is cooled before it reaches the environment. It is also known to return the heat obtained from the flue gas back into the combustion process, for instance for heating the combustion air. Such a heat exchanger suitable for this purpose is also known as regenerative heat exchanger.
In the combustion of organic matter or of household refuse which represents a mixture of organic and synthetic materials, besides carbon monoxide, nitrogen oxides and hydrocarbons, dioxins can result, and even in minimal amounts are toxic to humans. The xe2x80x9cdioxinsxe2x80x9d are a collection of cyclically halogenated aromatic polyethers. To this group pertain the cyclic ethers (furanes), as well as the cyclic diethers (the actual dioxins). As particularly toxic representatives the polychlorinated dibenzodioxin (PCDD) and the polychlorinated dibenzofurane (PCDF) should be mentioned here.
In order to eliminate the mentioned harmful substances, specifically acting catalysts can be used in the reaction zone of the flue gas pipe.
The EP 0 447 537 B1 describes a catalytic conversion system for the elimination of nitrogen oxides and dioxins from the flue gas of a garbage-incinerating plant. Here a standard DeNOx-catalytic converter, which working according to the known SCR-process of the selective catalytic reduction with the addition of a reducing agent, reduces nitrogen oxides to molecular nitrogen and water, and an oxidation catalytic converter which oxidizes dioxins are arranged in series. Both catalytic converters basically contain titanium dioxide TiO2, tungsten trioxide W03, vanadic pentoxide V2O5 and optionally molybdenum trioxide MoO3.
Also from DE 43-04 323 A1 a catalytic conversion system for the removal of nitrogen oxides NOx, carbon monoxide Co and dioxins is known. There in the flow direction of the flue gas of a combustion installation, a reduction or SCR-catalytic converter precedes an oxidation catalytic converter and the latter precedes a dioxin catalytic converter.
There are also known catalytic converters which, contrary to the SCR-catalytic converters, remove the nitrogen oxides contained in the flue gas without using separate reduction agents. These catalytic converters are generally defined as SCD-catalytic converters (SCD=Selective Catalytic Decomposition).
It is also known to thermally break down the dioxins contained in the flue gas by heating it in a reaction zone to a temperature over 800xc2x0 C.
Further from EP 0 502 443 B1 it is known to perform the decomposition of nitrogen oxides directly in a regenerative heat exchanger, by means of a SCR-catalytic converter arranged therein.
However it has been found that when using a heat exchanger arranged downstream of the reaction tone, the flue gas reaching the environment contains again dioxins in amounts which can not be dismissed.
Therefore it is the object of the invention to reliably avoid dioxin emissions into the environment in a flue gas pipe with a reaction zone and a subsequently arranged heat exchanger.
According to the invention this task is achieved by a flue gas pipe with a reaction zone for breaking down harmful substances contained in the flue gas and a heat exchanger for cooling the flue gas arranged downstream thereof, whereby the heat exchanger comprises a dioxin catalytic converter designed for breaking down dioxins.
The invention starts out from the concept that in a flue gas of a combustion installation at a temperature between 250 and 400xc2x0 C. a renewed formation of dioxins takes place through a so-called de novo synthesis. The mechanism of this de novo synthesis is described in detail in the VDI Reports No. 634, 1987, pages 557 to 584. Dioxins are formed in the presence of oxygen from organic compounds which have not burned and metal chlorides, whereby in a first stage available heavy metal components catalyze a chlorine formation from the mentioned metal chlorides. Since the compounds responsible for a de novo synthesis are present in the flue gas of a power plant running on fossil fuel or of a garbage incinerator even after the catalytic treatment of the flue gas takes place, each heat exchanger arranged downstream of the reaction zone, as a result of the performed temperature reduction, represents a potential source for renewed dioxin formation. Particularly since, as a rule, in a subsequent beat exchanger the flue gas is cooled to a temperature of less than 400xc2x0 C.
However such a renewed formation of dioxins in the flue gas after the already performed treatment of harmful substances is safely avoided when the heat exchanger arranged downstream of the reaction zone contains a dioxin catalytic converter equipped for the breakdown of dioxins. In this way on the one hand the renewed formation of dioxins is avoided when the temperature drops, and on the other hand already formed dioxins are decomposed into inoffensive substances.
As a dioxin catalyst for instance a catalyst can be used which contains TiO2, R WO3, V2O5 and optionally MoO3. The content of V2O5 has thereby to be adjusted to the conditions of the flue gas. A catalyst in plate, honeycomb or pellet form is suitable.
It is particularly cost-effective and technically simple when the heat exchanging element of the heat exchanger is formed by the dioxin catalytic converter itself, and the heat exchange takes place on the same. For this purpose a correspondingly coated honeycomb or plate-shaped catalyst is particularly suited, which due to the large specific surface has also a high heat storage capacity.
The efficiency of the combustion installation increases when the used heat exchanger is a regenerative heat exchanger. Such a regenerative heat exchanger on one hand extracts the heat front the flue gas and on the other band returns the extracted heat back into the combustion process of the combustion installation. This can take place for instance by heating the combustion air.
The dioxin content of the flue gas can be further reduced when the reaction zone comprises an additional dioxin catalytic converter or a dioxin adsorber. For instance a honeycomb or plate-shaped catalyst of an already mentioned composition on a titanium dioxide basis is suitable. A dioxin catalytic converter in the form of charge or in the form of pellets is also conceivable.
Particularly the arrangement in series of a dioxin catalytic converter with a DeNOx-catalytic converter in the reaction zone offers advantages in that, under substantially identical reaction conditions, it is possible to remove simultaneously nitrogen oxides and dioxins from the flue gas. Since both catalytic*converters have a similar chemical composition, it is possible to provide a single catalyst carrier for both catalytic converters.
The invention also comprises a method for flue gas treatment, whereby a flue gas of a combustion installation flows first through a reaction zone for the removal of harmful substances and is subsequently passed over a downstream-arranged heat exchanger for cooling, whereby the flue gas is contacted in the heat exchanger with a first dioxin catalytic converter breaking down the dioxins.
A particularly effective decomposition of dioxins and nitrogen oxides can be achieved when the flue gas in contacted in the reaction zone first at a temperature of 300 to 500xc2x0 C. with a DeNOx catalytic converter and/or with an additional second dioxin catalytic converter and subsequently is passed for cooling over the heat exchanger with the first dioxin catalytic converter.